Rust and Swift (ii)

Basic types and the syntax around them.

September 06, 2015 (updated March 12, 2016)Filed under Tech#programming languages#rust#rust-and-swift#swiftMarkdown source

I am reading through the Swift book, and comparing it to Rust, which I have also been learning over the past month. As with the other posts in this series, these are off-the-cuff impressions, which may be inaccurate in various ways. I’d be happy to hear feedback! Note, too, that my preferences are just that: preferences. Your tastes may differ from mine. (See all parts in the series.)


At first blush, I find the extra syntax around optionals in Swift more confusing than helpful. I think this comes down to my preference for a more Python-like approach: “Explicit is better than implicit” and “There should be one– and preferably only one –obvious way to do it” both militate against the multiple different ways you can handle optional values in Swift. Optional types are created in one of two ways:

  • with the ? operator on a type definition, creating an explicitly wrapped type which must be checked in some way.
  • with the ! operator on a type definition, creating an “implicitly unwrapped optional” by forcibly unwrapping it (and creating a runtime error if the optional is empty)

After creating an optional, you can get at its contents by:

  • using the if let or while let constructs to bind the optional value’s non-nil value for a block
  • using the ! operator on a variable name, explicitly unwrapping it (and creating a runtime error if the optional is empty)

By contrast, in Rust you always have to explicitly unwrap the item, using the unwrap method or pattern matching. There are no implicitly unwrapped types. Moreover, there is no special syntax around creating optional types in Rust: you just declare them with an Option type or another type that impls the Option behavior. The “shortcut” behavior around error handling, try!, isn’t special syntax, but application of another standard language construct (in this case, a macro).

The discussion of assert in the Swift book re-raises the question about the global namespace:

“You write an assertion by calling the global assert(_:_:) function.”

This continues to suggest strongly that Swift does in fact have a true global namespace, not an automatically-imported prelude. That can make a big difference for applications in certain spaces (e.g. systems programming), when you might have good reason to want to replace the standard library’s approach with a different one. (See Rust’s #[no_std] docs and the related RFC.)

Edit: “strongly suggests” or no, I have now been reliably informed that I was mistaken—and am happy to have been wrong here. As in Haskell, these functions are implicitly imported and belong to the Swift module.

In Rust, assert! is a macro, not a function, which is an interesting but perhaps not especially important distinction in this particular case. (It might be, though; I’d have to see the implementation of each to see how they play out differently.)

In any case, this also highlights another large difference between the two: testing is front and center in Rust, and barely receives a mention so far in the Swift book (and isn’t in the table of contents). Having language-level support for testing is a big deal.

Language tour and first chapter of the language guide down, my sense is that Swift is a substantially better language than C or C++ (and presumably than Objective C, but since I don’t know that language I can’t speak to it) for app design, but that Rust is a better language yet. Both a far more modern than their predecessors, but they approach the same problems in surprisingly different ways, relatively similar syntax notwithstanding. So far, I like the Rust approach better.

In particular, more syntax is not my preferred way to tackle these things. Providing good language constructs and primitives on which to build seems better in many ways:

  • It substantially reduces the cognitive load for the developer, by keeping the number of constructs small and simply varying how they are applied.
  • It increases the quality of those primitives, because it forces the language deadness to make sure they actually address the full problem space.
  • It lets developers approach the same problem in ways the language design team may not have anticipated, and over time the community may find shared conventions that improve on the std approach, and nothing has to change in the language spec (or the compiler!) to adopt those changes.
  • In general, then, it makes change much easier to manage, and change can be community-driven rather than requiring the language design team to manage it.1


  1. This may of course be intentional on Apple’s part with Swift. Maintaining tight control over its tooling is very typical of modern Apple.